Patient specific auto-flowrate control

ABSTRACT

A respiratory assistance system can provide high flow therapy to patients. The respiratory assistance system can include a patient interface that can deliver a gas flow to a patient and a gas source that can drive the gas flow towards the patient interface at an operating flow rate. The system can include a controller for controlling the operating flow rate of the gas. The controller can apply multiple test flow rate values in a range as the operating flow rate. For each of the test flow rate values, the controller can measure a patient parameter. The controller can determine a new flow rate value based on the measured patient parameters. Patient parameters can include respiration rate, work of breathing, or any other parameters related to the respiratory circuit.

INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/521,874, filed Apr. 25, 2017, which is a U.S. National Phase ofInternational Patent Application No. PCT/NZ2015/050179, filed Oct. 28,2015, now U.S. Pat. No. 10,722,670, which claims the priority benefit ofU.S. Provisional Application No. 62/069,760, filed Oct. 28, 2014, theentirety of which is hereby incorporated by reference herein. Any andall applications for which a foreign or domestic priority claim isidentified in the Application Data Sheet as filed with the presentapplication are hereby incorporated by reference under 37 CFR 1.57.

BACKGROUND

A respiratory assistance system including a humidification apparatus maybe used to deliver heated and humidified respiratory gases to a patientthrough a conduit and a patient interface. The respiratory assistancesystem can provide a number of therapies for patients requiringrespiratory support. One of the therapies includes providing a high flowtherapy. In high flow therapy, the respiratory support system deliversrelatively high flows of gases through an interface, such as an unsealednasal cannula. The flow rate of gases can be in the range of 5 L/min to120 L/min for patients.

SUMMARY

Certain aspects, advantages and novel features of the present disclosureare described herein. It is to be understood that not necessarily allsuch advantages may be achieved in accordance with any particularembodiment of the present disclosure. Thus, the features, aspects, andadvantages of the present disclosure may be embodied or carried out in amanner that achieves or selects one advantage or group of advantages astaught herein without necessarily achieving other advantages as may betaught or suggested herein.

In certain embodiments, a respiratory assistance system can include apatient interface that can deliver a gas flow to a patient. Therespiratory assistance system can also include a gas source that candrive the gas flow to the patient interface at an operating flow rate.The respiratory assistance system can also include a hardware processor.The hardware processor can apply a plurality of test flow rate values ina range as the operating flow rate. The hardware processor can furthermeasure at least one patient parameter corresponding to each of theplurality of test flow rate values. The hardware processor can alsodetermine a new flow rate value based at least in part on the measuredat least one patient parameter. In some embodiments, the hardwareprocessor changes the operating flow rate to the new flow rate value.

The system of the preceding paragraph can have any sub-combination ofthe following features: wherein the hardware processor can furtherdetermine a rate of change in the measured at least one patientparameter as a function of the change in the applied plurality of testflow rate values; wherein said determining the new flow rate furtherincludes determining where in the range of the plurality of test flowrate values does the rate of change approach zero; wherein saiddetermining the new flow rate can further include determining a minimumor a maximum value of the at least one patient parameter measured foreach of the plurality of test flow rate values; wherein the at least onepatient parameter comprises a respiratory rate and wherein the hardwareprocessor can measure the respiratory rate based at least in part on oneor more of the following sensor measurements: pressure fluctuations,flow rate fluctuations, blower fans speed, blower motor power, blowermotor torque, expired CO₂ fluctuations, transcutaneous CO₂ fluctuations,expired patient temperature, EMG signals, Edi signals, impedancepneumography, respiratory inductance plethysmography, acoustic sensing;wherein the at least one patient parameter can include a work ofbreathing indicator; wherein the patient parameter can include anexpiratory CO₂ concentration indicator; wherein the patient parametercan include a thoracoabdominal asynchrony indicator; wherein thehardware processor can wait a predetermined time period after the changein the operating flow rate before measuring the patient parameter;wherein the range can include one of the following: 0.5-1.5; 1-2;1.5-2.5; 2-3; 0.5-4; 1-3; 2-3; and 1-4 (in L/min/kg); wherein the rangecan include one of the following: 5-120; 5-60; 20-120; 20-60; 60-120;and 50-70 (in L/min) for patients; wherein changing the operating flowrate can include changing an amount of power delivered to the gassource; wherein the gas source can include a blower; wherein thehardware processor can generate an alarm value based at least in part onthe measured at least one patient parameter; wherein the hardwareprocessor is can determine that the measured at least one patientparameter is insensitive to the change in flow rate and generate analarm based on said determination; wherein the patient interface caninclude any of the following: unsealed nasal cannula, sealed nasalcannula, sealed nasal mask, or a full face mask; wherein the gas sourcecomprises a flow meter, a blender, or a flow mode from a ventilator; andwherein the plurality of test flow rates are applied responsive to auser input.

In some embodiments, a method for delivering gas to a patient caninclude delivering a gas flow to a patient via a patient interface. Themethod can further include driving the gas flow from a gas source to thepatient interface at an operating flow rate. The method can also includeapplying a plurality of test flow rate values in a range as theoperating flow rate. Further, the method can include measuring at leastone patient parameter corresponding to each of the plurality of testflow rate values. The method can further include determining a new flowrate value based at least in part on the measured at least one patientparameter. In some embodiments, the method can include changing theoperating flow rate to the new flow rate value.

The method of the preceding paragraph can have any sub-combination ofthe following features: determining a rate of change in the measured atleast one patient parameter as a function of the change in the operatingflow rate; determining where in the range of the plurality of test flowrate values that the rate of change approaches zero; wherein the atleast one patient parameter can include a respiratory rate; wherein theat least one patient parameter can include a work of breathingindicator; wherein the patient parameter can include a thoracoabdominalasynchrony indicator; wherein the range comprises one of the following:0.5-1.5; 1-2; 1.5-2.5; 2-3; 0.5-4; 1-3; 2-3; and 1-4 (in L/min/kg);wherein the range can include one of the following: 5-120; 5-60; 20-120;20-60; 60-120; and 50-70 (in L/min); wherein the patient interface caninclude any of the following: unsealed nasal cannula, sealed nasalcannula, sealed nasal mask, or a full face mask; wherein said pluralityof test flow rate values are received from a user; generating an alarmbased on the measured at least one patient parameter; and wherein thegas source comprises any of the following: a blower, a flow meter, aflow mode from a ventilator, and a blender.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will be described with respect to the following figures,which are intended to illustrate and not to limit the preferredembodiments.

FIG. 1 illustrates a diagram of a respiratory assistance systemaccording to an embodiment of the present disclosure.

FIG. 2 illustrates a block diagram of a control system interacting withand/or providing control and direction to components of a respiratoryassistance system according to an embodiment of the present disclosure.

FIG. 3 illustrates a block diagram of a controller according to anembodiment of the present disclosure.

FIG. 4 illustrates a flow chart of a method for respiratory assistancesystem parameter determination according to an embodiment of the presentdisclosure.

FIG. 5 illustrates a flow chart of a method for determination of flowrate value based on work of breathing according to an embodiment of thepresent disclosure

FIG. 6 illustrates an example graph corresponding to measurements ofwork of breathing indicators versus flow rate according to an embodimentof the present disclosure.

FIG. 7 illustrates a flow chart of a method for determination of flowrate value based on thoraco-abdominal asynchrony according to anembodiment of the present disclosure

FIG. 8 illustrates a flow chart of a method for determination of flowrate value based on respiration rate according to an embodiment of thepresent disclosure.

DETAILED DESCRIPTION

A respiratory assistance system including a humidification apparatus maybe used to deliver heated and humidified respiratory gases to a patientthrough a conduit and a patient interface. The respiratory assistancesystem can provide a number of therapies for patients requiringrespiratory support. One of the therapies includes providing a high flowtherapy. In high flow therapy, the respiratory support system deliversrelatively high flows of gases through a nasal interface, which may beunsealed. The flow of gases can be in the range of 5 L/min to 120 L/min.In some embodiments, the flow of gases can be in the range of 10 L/minto 120 L/min. In some embodiments, the flow of gases can be in the rangeof 20 L/min to 120 L/min. In some embodiments, the flow of gases is inthe range of 30 L/Min to 50 L/min. In some embodiments, the flow rate ofgases can be as high as 60 L/min. In some embodiments, the flow rate isgreater than 60 L/min, but less than 120 L/min. In other embodiments,the flow rate is 120 L/min or higher. The respiration assistance systemcan adjust the flow rate of gases during the treatment through a controlsystem. A discussion of high flow therapy and how the flow rate can bechanged in a respiratory assistance system can be found in PCT Pub. No.WO 2015/033288, titled “Improvements to Flow Therapy”, which is herebyincorporated by reference in its entirety.

The flow rate in the high flow therapies may be a function of patientcondition and can vary during the treatment. A clinician or patient maynot be able to determine the set value of the flow rate for therespiratory assistance system to provide the optimal therapy andcomfort. Care providers often do not know proper flow rates forparticular patients and tend to set flow rates too low or too high to beclinically optimal. Care provides also often do not know how to gaugethe effectiveness of the therapy or how long they should wait todetermine effectiveness. Many care providers also may not understand howand when to wean a patient off from high flow therapy.

Accordingly, the present disclosure provides methods and systems fordetermining an optimal flow rate for a given patient. The methods can beperformed by a control system of the device, although certain steps canalso be performed by a clinician in communication with the controlsystem. In some embodiments, the respiratory assistance system discussedbelow includes a control system implemented using a controller fordetermining a set value for the flow rate. The control system candetermine the set value of the flow rate and automatically update itover the time of therapy or based on change in patient conditions.Alternatively, the control system can provide an indication to aclinician to reevaluate the flow rate for the patient or perform one ormore other steps to determine a flow rate. Thus, in some embodiments,the control system can advantageously improve the efficacy of the highflow therapy and reduce the probability of the patient requiring moreinvasive treatment such as invasive mechanical ventilation. A flowratecontrol method for high flow respiratory therapy may help in a patientspending less time with a flow rate set too high or too low for theirimmediate breathing support requirements over the course of the therapy.

Flow rate is likely to affect a number of physiological and clinicalparameters including work of breathing, end tidal CO₂, respiration rate,thoraco-abdominal phase, and other parameters of clinical relevance. Inan embodiment, the control system discussed below can generate anindication of the set flow rate for display to a physician. The controlsystem can also automatically change the flow rate. In some embodiments,the control system can warn the clinician if the therapy is notefficacious for the particular patient based on the sensitivity orinsensitivity of clinical and physiological parameters to the flow rate.The control system described below can automatically control high flowrespiratory therapy flow rates based at least on one or more of thefollowing parameters: respiratory rate, end tidal CO₂, thoraco-abdominalasynchrony, and work of breathing.

In some embodiments, the control system's automatic determination of theset value for the flowrate can improve weaning times and reduce hospitallengths of stay. Patients can spend less time at flow rates higher thanwhat is necessary and discontinuation of therapy can be identifiedearlier. The control system can also assist in faster identification oftherapy success or failure. For example, it may be advantageous to knowthat high flow therapy is not working on a particular patient earlierrather than later. In an embodiment, the control system can compare thephysiological parameters of the patient as a function of flow rate toexpected predetermined parameters for determining effectiveness of thetherapy.

Respiratory Assistance System

FIG. 1 shows a schematic of an example respiratory assistance system100. As illustrated, the respiratory assistance system 100 includes ahumidification apparatus 104, a gases source 102, a patient interface116, and an inspiratory conduit 106 that can transport respiratory gasesfrom the humidification apparatus 104 to the patient interface 116. Thepatient interface 116 can include any one of the following interfaces:an unsealed nasal cannula (as shown), sealed nasal cannula, sealed nasalmask, or a full face mask (with, for example, a castle port). Otherexamples of respiratory systems including nasal cannula are discussedmore in detail in PCT Publication WO 2015/33288, titled “Improvements toFlow Therapy”, incorporated herein by reference in its entirety. In someembodiments, the gases source 102 and the humidification apparatus 104may be co-located, within the same housing, and/or comprise a singleapparatus. In some embodiments, the humidification apparatus 104 may notbe included in the respiratory assistance system 100. Accordingly, gasesmay flow directly from the gases source 102 to the patient interface116. In some embodiments, a headgear 150 mechanically supports thepatient interface 116 to the patient.

The gases source 102 may include a flow source 124 that can create aflow of respiratory gases to be provided to the humidification apparatus104. In an embodiment, the flow source 124 is a blower. However, theflow source 124 is not limited to a blower and can include a flow meter,a blender, flow mode from a ventilator, or any other flow generatingdevice. Other flow sources known to those of skill in the art can alsobe used with any of the embodiments of the present disclosure as furtherdiscussed below. In an embodiment, the flow source 124 can include a fanand a motor. In some embodiments, the gases source 102 may comprise aninlet 122 through which ambient air is drawn into the gases source 102.In some embodiments, instead of drawing ambient air, the inlet 122 canbe connected to a dry gas source, for example, a gas canister or tank.In some embodiments, the gases source 102 may include a controller 126that can control the operation of the flow source 124. For example, thecontroller 126 can execute or implement a control system described morein detail below to control operations of the flow source and theassociated flow rate of gases. The control system can, for example insome embodiments that use a blower as a flow source, determine an amountof power delivered to the blower. The fan or motor speed may depend onthe amount of power. In some embodiments, the gases source 102 maycomprise a user interface 132 configured to provide informationregarding user input to the controller 126. The controller 126 may beconfigured to control the operation of the blower or other flow source124 based on information provided by the user interface 132 and/or basedon other information, for example but not limited to, feedback from theflow source 124, such as from a sensor associated with the flow source124 as described more in detail below with respect to the controlsystem. The user interface 132 may include buttons, knobs, dials,switches, levers, touch screens, speakers, displays and/or other inputor output elements.

As discussed above, the humidification apparatus 104 may be optionallyincluded in the respiratory assistance system 100 but is not necessaryin every embodiment. The humidification apparatus 104 may include ahumidification chamber 134 and a chamber heater 136. The humidificationchamber 134 may be configured to hold a volume of water W or othersuitable liquid. The chamber heater 136 may be configured to heat thevolume of water W and respiratory gases within the humidificationchamber 134, which may increase the temperature of the respiratory gasesand may create vapor from the volume of water W that is taken up by therespiratory gases. In some embodiments, the humidification chamber 134may comprise a chamber inlet (not shown) and a chamber outlet 112. Insome embodiments, the inspiratory conduit 106 may be configured to beconnected to the chamber outlet 112, such that heated and humidifiedrespiratory gases may be transported by the inspiratory conduit 106 fromthe humidification chamber 134 to the patient interface 116 and thendelivered to a patient P.

In some embodiments, the humidification apparatus 104 may comprise acontroller 130 configured to control the operation of the chamber heater136 and a flow generator, such as the flow source 124. In theembodiments which do not include the humidification apparatus 104, therespiratory assistance system 100 can use controller 126 instead ofcontroller 130. In some embodiments, the humidification apparatus 104may include a user interface 140 to provide information regarding userinput to the controller 130. In some embodiments, the humidificationapparatus 104 may include an ambient sensor 146. The ambient sensor 146may measure a characteristic of the ambient air near the location of theambient sensor 146, such as a temperature of the ambient air. Thecontroller 130 may receive information regarding a characteristic of theambient air near the location of the ambient sensor 146 from the ambientsensor 146. The controller 130 or 126 can control the operation of theflow source 124 based on information provided by the user interface 140,based on information provided by physiological sensors, and/or based onother information, for example but not limited to, feedback from thechamber heater 136, such as from a sensor associated with the chamberheater 136. In particular, the controller 130 may be configured todetermine an amount of power, or a power duty cycle, to provide to thechamber heater 136 such that the chamber heater 136 delivers a desiredamount of heat to respiratory gases and the volume of water W within thehumidification chamber 134.

In some embodiments, the respiratory assistance system 100 may includean outlet sensor 110 that is associated with the chamber outlet 112. Theoutlet sensor 110 may be located at, in, or on the chamber outlet 112,at, in, or on the inspiratory conduit 106 near the connection betweenthe chamber outlet 112 and the inspiratory conduit 106, or at, in, or onanother suitable location. The outlet sensor 110 may measure acharacteristic of respiratory gases flowing past the location of theoutlet sensor 110, such as a temperature of the respiratory gases. Thecontroller 130 may receive information regarding a characteristic ofrespiratory gases flowing past the location of the outlet sensor 110from the outlet sensor 110. The controller 130 may be configured tocontrol the operation of the chamber heater 136 based on informationprovided by the outlet sensor 110, instead of or in addition to othersources of information as previously described.

Control System

FIG. 2 illustrates a block diagram of an embodiment of a control system220 that can detect patient conditions and control operation of therespiratory assistance system 100 including the gas source 124. In anembodiment, the control system 220 manages flow rate 232 of the gasflowing through the respiratory assistance system 100 as it is deliveredto a patient. The control system 220 can increase or decrease the flowrate by controlling a motor speed of the blower or a valve in a blender.The control system 220 can automatically determine a set value or apersonalized value of the flow rate for a particular patient asdiscussed below. In some embodiments, the flow rate can be optimized bythe control system 220 to improve patient comfort and therapy.

The control system 220 can also generate audio and/or visual outputs234. For example, the respiratory assistance system 100 can include adisplay 330 (see FIG. 3 ) which may further include a speaker. Thedisplay 330 can indicate to the physicians any warnings or alarmsgenerated by the control system 220. The display 330 can also indicatecontrol parameters that can be adjusted by the physicians. For example,the control system 220 can automatically recommend a flow rate for aparticular patient. The control system 220 can also generate recoverystate of the patient and send it to the display.

In some embodiments, the control system 220 can change a temperature setpoint 230 of one of the heating elements, such as chamber heater 136, tocontrol the output conditions of the gas delivered to the patient. Thecontrol system 220 can also change the operation or duty cycle of theheaters described above.

The control system 220 can determine outputs 230-234 based on one ormore received inputs 202-210. The inputs 202-208 can correspond tosensor measurements received automatically by the controller 128 or 132.In the illustrated embodiment, the control system 220 receives sensorinputs corresponding to thoraco-abdominnal asynchrony (TAA) sensorinputs 202, respiration rate sensor inputs 204, work of breathing sensorinputs 206, and CO₂ sensor inputs 208 and or other sensors (pressuresensor, ambient sensor) in the respiratory assistance system 100described above. In an embodiment, the control system 220 can alsoreceive inputs from user 210 or stored values in a memory 324. Thecontrol system 220 can dynamically adjust flow rate 232 for a patientover the time of their therapy. In an embodiment, the control system 220can continuously detect system parameters and patient parameters.

Thoraco-Abdominal Asynchrony

In a healthy patient, the abdomen and the rib cage move in synchronywhen breathing. Thoraco-abdominal asynchrony (TAA) occurs when there isasynchronous movement between the ribcage and abdomen during breathing.Accordingly, TAA is the non-coincident motion of the abdomen and ribcage and can be an indication of respiratory distress. Higher phasedifference between the movement of the abdomen and rib cage can indicatea greater level of respiratory distress. In some embodiments, thecontrol system 220 can change the set value of the flow rate of gases toreduce the phase difference. TAA sensor measurements can includemeasurements from any device or sensors that can detect movement orelectrical signal from the abdomen and rib cage. For example, in anembodiment, sensor measurements can include measurements from arespiratory inductance plethysmography or skin mounted strain gauges.Further, the measurements can include electrical activity of thediaphragm and other muscles. This electrical activity can be measured byelectromyography (EMG), sEMG, EDI, or electrical impedance tomography(EIT) sensors.

Work of Breathing (WOB)

Work of breathing can correspond to a measure of the effort required toinspire air into the lungs and can be an indication of a number ofdifferent breathing disorders. A high work of breathing is uncomfortablefor the patient, can lead to elevated CO₂ levels, and can result in apatient being escalated to more invasive care such as invasivemechanical ventilation. Therefore, in some embodiments, the controlsystem 220 can determine a flow rate that reduces work of breathing. WOBcan be measured as energy in Joules spent by a patient to breath over aminute. WOB may be difficult to detect or measure directly from sensors.Accordingly, in some embodiments, the control system 220 may indirectlymeasure an indication corresponding to WOB from one or more sensorinputs 206. For example, the control system 220 can indirectly measureWOB from EMG measurements, EDI measurements, respiratory inductanceplethysmography (RIP), minute ventilation, expiratory time,pressure-rate product, respiration rate, TAA, pressure-time product orCO₂ measurements. EMG can correspond to the magnitude of the electricalsignal from the brain to the diaphragm muscles. When the brain signalsto breathe more or respiratory distress exists, the EMG signals canincrease. RIP measures the movement of the chest wall and/or abdomen andoutputs a voltage. Higher voltage can indicate a larger movement of thechest wall and/or abdomen and therefore indicate a higher WOB.

In an embodiment, the control system 220 can store EMG and RIP valuesacross a large group of patients for comparison between a currentpatient and average values. The control system 220 can determine outputparameters based on the average patient values. However, in someinstances, patient conditions can vary substantially. Thus, in someembodiments, it may be advantageous to monitor a particular patient todetermine output parameters for that patient. For example, the controlsystem 220 can use changes in either EMG or RIP or both to determinewhether a change in flow rate increases or decreases WOB. Further,changes in calculated parameters such as minute ventilation, expiratorytime, pressure-rate product and/or pressure-time product can also beused to indicate a work of breathing response.

Respiration Rate

Respiratory rate can be an important indicator of patient condition. Anabnormal respiratory rate has been shown to be a predictor of seriousevents such as cardiac arrests and escalation to high levels of care.Respiratory rate can thus provide an indication of deterioration orimprovement in patient condition. Respiration rate may also be relatedto work of breathing.

CO2 Measurements

Patient respiration can also be measured through the use of capnographyto determine partial pressure of CO2 in the respiratory gases. Theconcentration or partial pressure of CO₂ in the gas exiting the airwayat the end of expiration is called end tidal CO₂. Measurement of CO₂(capnography) is common during anaesthesia and intensive care and it isusually presented as a graph of expiratory CO₂ plotted against time, or,less commonly, expired volume. The gas expired at the end of expirationcan be an indirect but relatively accurate measure of the CO₂ partialpressure in arterial blood. Capnography therefore provides informationon patient condition, including, for example, CO₂ production, lungperfusion, breathing patterns (including respiration rate), alveolarventilation, and CO₂ elimination.

In some embodiments, the control system 220 can use CO₂ measurements orindications of CO₂ measurements 208 to determine a set value of flowrate 232 for a particular patient. Direct measurements of end tidal CO₂accurately during high flow therapy may be difficult because the cannulaflow may dilute and flush the expiratory breath with fresh gas. A methodof measuring content of gases from diluted measurements in exhaledbreath is described in U.S. Prov. No. 62/046,052 titled “Exhaled GasMeasurement Compensation During High Flow Respiratory Therapy”. In someembodiments, the control system 220 can use relative values of CO₂ todetermine a set value of flow rate as discussed below. The controlsystem 220 can adjust the flow rate to lower the CO₂ volume in theexpiration. In an embodiment, the control system 220 can adjust the flowrate until changing it any more results in an increase in the lower endtidal CO₂ volume. In some embodiments, the control system 220 can adjustthe flow rate to maximize end tidal oxygen (O₂) volume.

Controller

The control system 220 can include programming instructions fordetection of input conditions and control of output conditions. Theprogramming instructions can be stored in a memory 324 of the controller126 and/or 130 as shown in FIG. 3 . In some embodiments, the programminginstructions correspond to the methods, processes and functionsdescribed herein. The control system 220 can be executed by one or morehardware processors 322 of the controller 126 and/or 130. Theprogramming instructions can be implemented in C, C++, JAVA, or anyother suitable programming languages. In some embodiments, some or allof the portions of the control system 220 can be implemented inapplication specific circuitry 328 such as ASICs and FPGAs.

As illustrated in FIG. 2 , the control system 220 can receive inputsfrom multiple components of the respiratory assistance system 100. Notall of the inputs 202-210 shown in FIG. 2 may be present. The inputs 202to 210 and the outputs 230 to 234 may not necessarily be present in allembodiments. For example, in some embodiments, the control system 220may only receive the EMG input 206 and generate a flow controlmeasurement 232. Depending on the configuration, some of the componentscorresponding to the inputs may not be included in the respiratoryassistance system 100. Lack of input itself can be used by the controlsystem 220 to determine the input or system conditions.

FIG. 3 illustrates a block diagram of an embodiment of a controller 126or 132. The controller can include a hardware processor 322 that canexecute the instructions stored in a memory 326. In an embodiment, thecontrol system 220 is stored as programming instructions in the memory326. The controller can also include circuits 328 for receiving sensorsignals. The controller can further include a display 330 fortransmitting status of the patient and the respiratory assistancesystem. The display 330 can also show warnings. The controller can alsoreceive inputs from the display.

Patient Parameter Versus Respiratory Assistance System Parameter

FIG. 4 illustrates a flow chart of an embodiment of a method 400 forselecting a set value of a parameter of the respiratory assistancesystem. The parameter can include one or more of heater temperature,flow rate, heating element current. In some embodiments, the set valueis selected by the control system 220 to optimize patient comfort andtherapy. The method 400 can be implemented by any of the systemsdescribed herein. In an embodiment, the method 400 is implemented by thecontrol system 220.

In an embodiment, the method 400 begins at block 402. The control system220 can adjust a respiratory assistance system parameter over apredetermined range. As discussed above, one of the system parametersadjusted by the control system 220 can include a flow rate parameter.The control system 220 can, for example, increase the motor speed of theblower when a blower is used as the flow source 124 to increase the flowrate of gases through the respiratory assistance system 100. The controlsystem 220 can measure one or more patient conditions in response to thechange the system parameter.

In some embodiments, the control system 220 can wait for a predeterminedtime period before measuring the patient parameter. For example, thecontrol system 220 can wait for the patient to reach steady state beforemeasuring the patient parameter. The wait time can be less than 10minutes or greater than or equal to 10 minutes. In an embodiment, thecontrol system 220 waits until the patient parameter stops changingsignificantly after the change in flow rate. The patient parameter canbe obtained from one or more sensor measurements discussed above. In anembodiment, the patient parameter is directly obtained from the sensorsby the control system 220. In other embodiments, the patient parameteris indirectly deduced or calculated by the control system 220 from thesensor measurements. The patient parameters can include work ofbreathing, end tidal CO₂ volume, respiration rate, phase synchrony, andany other parameters indicating a patient's physiological condition. Atblock 406, the control system 220 can store the measured patientparameter in the memory. The control system 220 can also storeadditional parameters of patient and/or system in the memory andassociate it with the measured patient parameter. Accordingly, thecontrol system 220 can store the state of the patient and therespiratory assistance system 100 in conjunction with the measuredparameter. The control system 220 can adjust the system parameter againand repeat the measurement and storing steps of blocks 404 and 406respectively. Accordingly, the control system 220 can sweep through thevalues in a predetermined range for the system parameter and store thecorresponding patient parameters in response to the change in the systemparameter.

In some embodiments, the control system 220 selectively identifiesvalues within the range. For example, the control system 220 can use abinary search, start from two end points and select in the middle and soon. The control system 220 can also start from previously stored setvalues and increase or decrease the system parameter from the stored setvalue. A patient may notice big changes in the system parameters. Forexample, the patient can notice the big changes in the flow rate.Accordingly, in some embodiments, the control system 220 changes theparameters gradually and may start from the stored set value. The storedset value may also correspond to statistical average over time.

At block 408, the control system 220 can analyse the stored patientparameters as a function of the change in the system parameter. Based onthe analysis, the control system 220 can determine a set value for thesystem parameter. In an embodiment, the control system 220 can determinea derivative of a function corresponding to the stored patient parameterversus system parameter. The control system 220 can determine a pointwhere the derivative of the function is zero. This point can be used asthe set value for the patient parameter. Depending on the patientparameter, the point may correspond to either maximum or minimum valuesof the function. The control system 220 can also determine boundaryconditions so that the set value does not fall outside of apredetermined range. In some embodiments, at block 410, the controlsystem 220 can receive additional parameters corresponding to thepatient conditions and/or system conditions. Additional parameters mayinclude patient characteristics, such as age, sex, weight, awake orasleep and the like. System parameters may include time of day, type oftherapy selected, and the like. The control system 220 can use theseadditional parameters in determination of the set value.

At block 412, the control system 220 can change the respiratoryassistance system parameter to the determined set value. For example,the control system can adjust the flow rate using the process 400discussed above. The process 400 can, in some embodiments,advantageously optimize system parameters for improved patient comfortand therapy. The control system 220 can run the process 400 periodicallyto adjust the system parameter. The period may be adjusted by thecontrol system 220 based on changes in the patient condition. Forexample, a child with bronchitis may go from severe to normal in amatter of days while a patient with a chronic lung disease may needtherapy for a month, or indefinitely with gradual improvement or declineand thus gradual adjustments in flow rate. The control system 220 canalso run the process 400 in response to detecting a change in thepatient condition or system condition. For example, the control system220 can run the process 400 when the patient falls asleep or wakes up.In some embodiments, the control system 220 can run the process inresponse to an input from a user, such as a physician or a patient. Thecontrol system 220 can run the process until the patient can come offthe respiratory assistance system 100.

Work of Breathing Versus Flow Rate

FIG. 5 illustrates a flow chart of an embodiment of a process 500 foroptimizing flow rate to minimize a patient's work of breathing. Theprocess 500 extends process 400 discussed above for a particular systemparameter (flow rate) with respect to a particular patient parameter(work of breathing). Accordingly, some of the discussion above withrespect to the process 400 can also apply to process 500. The process500 can be implemented by any of the systems described herein. In anembodiment, the process 500 is implemented by the control system 220.

The process 500 can begin at block 502 during system initialization. Thecontrol system 220 can set the initial flow rate of the respiratoryassistance system 100. The initial flow rate may be stored in thememory. The initial flow rate can be a function of the age and/or weightof a patient. For example, in adults, the initial flow rate may besomewhere in the range of 5 to 120 L/min. In an embodiment, the initialflow rate is 30 L/min for an adult patient. For children or neonatalpatients, the initial flow rate may be 1 L/min/kg of the child's weight.In some embodiments, the initial flow rate may be greater than 1L/min/kg, but less than or equal to 3 L/min/kg. In some embodiments, theinitial flow rate is greater than 3 L/min/kg. The initial flow rate mayalso be less than 1 L/min/kg. The initial flow rate can also be receivedby the control system 220 as a user input. The initial flow rate may beestimated by the control system 220 based on comparing a patientcharacteristic with predetermined values stored in the memory. Asdiscussed above with respect to the process 400, the process 500 can beinitiated by the control system 220 periodically or based on an event.Accordingly, the process 500 can also begin at block 504 for a periodicmeasurement.

At block 504, the control system 220 can receive sensor measurements todetermine the patient's work of breathing corresponding to the currentflow rate of the respiratory assistance system 100. The sensormeasurements may correspond to electrical signals from EMG probes orNAVA probes attached near the chest of the patient as discussed above.The control system 220 can determine a direct value of work of breathingbased on the received sensor measurements at block 506. In someembodiments, the control system 220 does not have to directly oraccurately measure the work of breathing. As discussed above, thecontrol system 220 can determine set value from the derivative of afunction representing the relationship between the patient parameter andthe system parameter. Thus, accurate or direct values may not benecessary as long as the values are relatively comparable. Accordingly,the control system 220 can use voltage measurements from the EMG sensorsas a proxy for work of breathing. Since these measurements are from thesame patient and the configuration is likely to not change betweenmeasurements, it may be advantageous in some embodiments to comparerelative measurements as a function of a change in system parameter.Comparing relative measurements may require less processing power.Furthermore, in some embodiments, proxy measurements like voltage orcurrent can be used by the control system 220 instead of directlymeasuring patient condition.

At block 508, the control system 220 can store work of breathingmeasurements or the corresponding proxy measurements. The control system220 can determine if an additional measurement needs to be made at block510. The determination may be based on whether there are more flowparameters that need to be checked by the control system 220.

For example, in some embodiments applicable to Neonatal patients orchildren, the control system 220 starts with an initial flow rate of 1L/min/kg and applies a range of flow rates up to 3 L/min/kg startingfrom 1 L/min/kg. In some embodiments, the control system 220 can applyany one of the following range of flow rates depending on the patient(in L/min/kg): 0.5-1.5; 1-2; 1.5-2.5; 2-3; 0.5-4; 1-3; 2-3; and 1-4. Insome embodiments, the ranges discussed above include the initial oroperating flow rate. In some embodiments, the range of flow ratesscanned may not include the initial or operating flow rate. The controlsystem 220 can increase the flow rate by 0.5 L/min/kg for each check atblock 512. The control system 220 can wait for a predetermined timeperiod before changing the flow rate. The control system 220 can alsowait for a predetermined time period after the changing the flow rateand before taking the measurement. Other increments, for example, 0.1,0.3 or 1, are also possible. Increments may also be a function ofpatient condition or system parameters. For each increment, the controlsystem 220 can perform steps 504 to 508. When the control system 220 hasreached 3 L/min/kg, it can stop the loop and proceed to block 514. Insome embodiments, the control system 220 can stop the loop if itdetermines an increasing or decreasing trend in the patient parameter.For example, the control system 220 can stop the loop when thederivative is zero or close to zero. The derivative can correspond tothe rate of change of the patient parameter with respect to the systemparameter.

As another example, for an adult patient, the control system 220 canstart from 5 L/min and increase from that starting point. The controlsystem 220 can also start from 20 L/min. In an embodiment, the controlsystem 220 can start from a flow rate between 5 L/min and 120 L/min. Thecontrol system 220 can increase the flow rates incrementally asdiscussed above until a predetermined limit or any other condition asdiscussed above has been satisfied. The predetermined limit can be aflow rate of 120 L/min or lower. In an embodiment, the predeterminedlimit is 60 L/min. The ranges and the initial rates discussed herein mayalso be a function of patient characteristics, such as age and weight.In some embodiments, the control system 220 can apply any one of thefollowing range of flow rates (in L/min) for adults: 5-120; 5-60;20-120; 20-60; 60-120; 30-50; and 50-70. In some embodiments, the rangesdiscussed above include the initial or operating flow rate. In someembodiments, the range of flow rates scanned may not include the initialor operating flow rate. Further, the flow rate parameters may alsodepend on a physiological parameter, such as a respiration rate, or theillness. In some embodiments, whether a patient is classified as anadult or a child may depend on age, weight, therapy, or illness. Somechildren may be classified as adults and administered the adult flowrates depending on their age and/or weight. The flow rates may also bechanged by a clinician or a patient using the user interfaces.

After the control system 220 collects all the patient measurements, itcan analyse the measurements at block 514. The control system 220 canselect a set value of the flow rate from the measurements by identifyinga flow rate with the smallest work of breathing measurement. An exampleset of collected measurements is illustrated in a graph format in FIG. 6. The control system 220 can select the flow rate 602 corresponding tothe minimum point for work of breathing 604 as shown in FIG. 6 . Thecontrol system 220 can set boundary conditions and not select a flowrate below a minimum rate 606 The control system 220 can also cap flowrate at a maximum rate 608 that may be set by the clinician or stored inthe controller. This limit may be based on a flow above which thepatient may feel discomfort, for example 120 L/min for adults and 3L/min/kg for neonatal patients and children. Higher flow rates can alsoincrease noise and pressure. Accordingly, based on the data collected bythe control system 220, it can select a set value of the flow rate atblock 516. The control system 220 can change the current value of theflow rate to the determined set value at block 518. Accordingly, thecontrol system 220 can optimize flow rate to reduce work of breathingand improve patient comfort. In some embodiments, the control system 220can reduce work of breathing by 50% by determining a set value for theflow rate particularized for a patient.

Thoraco-Abdominal Asynchrony Versus Flow Rate

FIG. 7 illustrates a flow chart of an embodiment of a process 700 foroptimizing flow rate to minimize thoraco-abdominal asynchrony (TAA). Theprocess 700 extends process 400 discussed above for a particular systemparameter (flow rate) with respect to a particular patient parameter(TAA). Accordingly, some of the discussion above with respect to theprocesses 400 and 500 can also apply to the process 700. The process 700can be implemented by any of the systems described herein. In anembodiment, the process 700 is implemented by the control system 220.

At block 702, the control system 210 can set an initial flow rate. Thecontrol system can measure TAA at block 704 for the current flow rate.TAA can correspond to the phase angle between the chest and abdominalmotion. As discussed above, the control system 220 can use relativemeasurements as indications of TAA. Relative measurements can includephase difference in the voltage recorded from the electrodes attached tothe different parts of the body of the patient. Accordingly, the controlsystem 220 can receive voltage measurements from sensors at block 704.The control system 220 can identify the phase difference from thevoltage measurements between two different sites of the patient's bodyat block 706. The control system 220 can store the phase difference atblock 708 in the memory. The control system 220 can determine if anadditional measurement is necessary for a new flow rate at block 710. Ifnecessary, the control system 220 can change the flow rate at block 712and repeat step 704 to 708. If the control system 220 determines that itdoes not require additional data, it can proceed to block 714.

The control system 220 can analyse the stored phase measurements atblock 714. As discussed above with respect to the work of breathing, thecontrol system 220 can identify a flow rate for which the phasedifference is the lowest in the stored measurements. The control system220 can used the identified value as the set value of the flow rate atblock 716. Furthermore, the control system 220 can change the currentvalue of flow rate of the respiratory assistance system 100 to the setvalue. Accordingly, the process 700 can be used by the control system220 to optimize flow rate for reduced asynchrony.

Respiration Rate Versus Flow Rate

FIG. 8 illustrates a flow chart of an embodiment of a process 800 foroptimizing flow rate to respiration rate (RR). The process 800 extendsprocess 400 discussed above for a particular system parameter (flowrate) with respect to a particular patient parameter (RR). Accordingly,some of the discussion above with respect to the processes 400, 500, and700 can also apply to the process 800. The process 800 can beimplemented by any of the systems described herein. In an embodiment,the process 800 is implemented by the control system 220.

Respiration rate can indicate a patient condition or a work ofbreathing. Accordingly, the control system can use the respiration rateto determine a set value of a system parameter, such as a flow rate.Furthermore, respiration rate is easier to measure than some of thepatient parameters discussed above. However, a function corresponding torespiration rate versus flow rate may be substantially monotonic. Forexample, when the flow rate is continuously increased, the respirationrate correspondingly decreases and heads towards zero. There areclinical situations where it is not desirable for the respiration rateto approach zero. However, the control system 220 can identify a rangeof flow rates where increasing the flow rate does not decrease therespiration rate. In some embodiments, the control system 220 canidentify a section where the rate of change of the respiration rate isreduced.

The process 800 can begin at block 802 with the control system 220setting an initial flow rate. At block 804, the control system 220 canreceive sensor measurements corresponding to respiration rate. In anembodiment, the sensor measurement is a plethysmographic signal. Othermeasurements for determining respiration rate are discussed above.Respiration rate may also be inputted via a user interface and receivedby the control system 220. In some embodiments, the control system 220can determine respiration rate based on the received sensor measurementsat block 806. The control system 220 can store the measured respirationrate in the memory at block 808.

The control system 220 can determine if additional measurements ofrespiration rate are needed with respect to flow rate at block 810. Forexample, the control system 220 can determine if the current flow rateor the last measured respiration rate is at or exceeded a boundarycondition. If the control system 220 determines that additionalmeasurements are needed, then the control system 220 can change the flowrate at block 812 and repeat steps 804 to 808. In the alternative, ifthe control system 220 determines that additional measurements are notrequired, the control system 220 can analyse the stored RR measurementsat block 814. In some embodiments, the control system 220 can stopadditional measurements when the rate of the change of respiration ratewith respect to flow rate approaches zero. As discussed above, thecontrol system 220 can use the decrease in the rate of change todetermine the set value of the flow rate at block 816. The decrease inthe rate of change can correspond to a minimum in work of breathing.Further, the control system 220 can change the current value of the flowrate to the determined set value at block 818.

While the processes 500, 700, and 800 are described separately, thecontrol system 220 can measure multiple physiological parameters at thesame time with the change in flow rate. Accordingly, the control system220 can use a combination of the steps of the processes 500, 700, and800 to determine a set value of the flow rate. In an embodiment, thecontrol system 220 can average the flow rates determined independentlyfrom the different processes.

Warnings

The control system 220 can also generate alarms or warnings based on themeasured physiological patient parameters. For instance, if therespiration rate exceeds or drops below an acceptable limit, the controlsystem 220 can generate an alarm for the display. Alternatively thecontrol system can generate alarms or warnings based on relativeinsensitivity of measured parameters to changes in flow. For example ifthe work of breathing is insensitive to flow this may indicate that thetherapy is less likely to be efficacious. In an embodiment, the controlsystem 220 can change the flow rate and determine that the work ofbreathing is not affected significantly by the flow rate change. Basedon the lack of correlation, the control system 220 can determine thatthe therapy may not be optimal for the patient.

Applications

The respiratory assistance system 100 with high flow therapy can be usedto provide support to patients in emergency rooms, intensive care units(ICU), the operating room (OR), other hospital areas or in-home. Inparticular, the respiratory assistance system 100 can be used to supporta patient under anaesthesia, during preoxygenation and postoperation.Using high flow therapy can have advantages in some embodiments becausethe patient can still communicate and the mouth is not covered by amask. Any time a patient requires intubation or endoscopy, the mouth maybe blocked and cannot be used for providing invasive air support.Accordingly, high flow therapy along with the nasal cannulaconfiguration of the respiration assistance system 100 can be used inthose situations to provide breathing support. The control system 220can determine work of breathing or other physiological parameters inthese cases and automatically determine a set value for flow rate. Whenpatients use the respiratory assistance system 100 in their homes, thecontrol system 220 can be used to adjust the set value of flow rate atthe initial stage. The patient can also measure their respiration rateand enter it using the controller.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise”, “comprising”, and thelike, are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense, that is to say, in the sense of“including, but not limited to”.

Reference to any prior art in this specification is not, and should notbe taken as, an acknowledgement or any form of suggestion that thatprior art forms part of the common general knowledge in the field ofendeavour in any country in the world.

The disclosed apparatus and systems may also be said broadly to consistin the parts, elements and features referred to or indicated in thespecification of the application, individually or collectively, in anyor all combinations of two or more of said parts, elements or features.

Where, in the foregoing description reference has been made to integersor components having known equivalents thereof, those integers areherein incorporated as if individually set forth.

Depending on the embodiment, certain acts, events, or functions of anyof the algorithms, methods, or processes described herein can beperformed in a different sequence, can be added, merged, or left outaltogether (e.g., not all described acts or events are necessary for thepractice of the algorithms). Moreover, in certain embodiments, acts orevents can be performed concurrently, e.g., through multi-threadedprocessing, interrupt processing, or multiple processors or processorcores or on other parallel architectures, rather than sequentially.

It should be noted that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications may be madewithout departing from the spirit and scope of the disclosed apparatusand systems and without diminishing its attendant advantages. Forinstance, various components may be repositioned as desired. It istherefore intended that such changes and modifications be includedwithin the scope of the disclosed apparatus and systems. Moreover, notall of the features, aspects and advantages are necessarily required topractice the disclosed apparatus and systems. Accordingly, the scope ofthe disclosed apparatus and systems is intended to be defined only bythe claims that follow.

What is claimed is:
 1. A method for delivering gas to a patient using arespiratory assistance system comprising a patient interface configuredto deliver a gas flow to the patient and a gas source configured todrive the gas flow to the patient interface at an operating flow rate,said method comprising: applying a plurality of test flow rate values ina range as the operating flow rate; measuring at least one patientparameter corresponding to each of the plurality of test flow ratevalues; determining a rate of change in the measured at least onepatient parameter as a function of the change in the applied test flowrate values; determining a new flow rate value based at least in part onthe measured at least one patient parameter; and changing the operatingflow rate to the new flow rate value.
 2. The method of claim 1, furthercomprising determining a value in the range of the plurality of testflow rate values at which the rate of change in the measured at leastone patient parameter approaches zero.
 3. The method of claim 1, whereinsaid determining the new flow rate value further comprises determining aminimum value or a maximum value of the measured at least one patientparameter for each of the plurality of test flow rate values.
 4. Themethod of claim 1, wherein the at least one patient parameter comprisesa respiratory rate and wherein the method further comprises measuringthe respiratory rate based at least in part on one or more of thefollowing measurements: pressure fluctuations, flow rate fluctuations, ablower fans speed, a blower motor power, a blower motor torque, expiredCO2 fluctuations, transcutaneous CO2 fluctuations, an expired patienttemperature, EMG signals, Edi signals, impedance pneumography,respiratory inductance plethysmography, or acoustic sensing.
 5. Themethod of claim 1, wherein the at least one patient parameter comprisesone or more of a work of breathing indicator, an expiratory CO2concentration indicator, or a thoracoabdominal asynchrony indicator. 6.The method of claim 5, wherein the work of breathing indicator comprisesa respiration rate of the patient, wherein the operating flow rate isselected such that the respiration rate of the patient is minimized orreduced.
 7. The method of claim 1, further comprising providing ahardware processor which is configured to wait a predetermined timeperiod after the change in the operating flow rate before measuring theat least one patient parameter.
 8. The method of claim 1, wherein therange comprises one of the following: 0.5-1.5; 1-2; 1.5-2.5; 2-3; 0.5-4;1-3; 2-3; or 1-4 (in L/min/kg), or 5-120; 5-60; 20-120; 20-60; 60-120;or 50-70 (in L/min).
 9. The method of claim 1, wherein said changing theoperating flow rate comprises changing an amount of power delivered tothe gas source.
 10. The method of claim 1, wherein said determining thenew flow rate value further comprises minimizing a work of breathing ofthe patient.
 11. The method of claim 1, further comprising generating analarm based on the measured at least one patient parameter.
 12. Themethod of claim 11, wherein said generating the alarm is in response todetermining that the measured at least one patient parameter isinsensitive to the change in the operating flow rate.
 13. The method ofclaim 1, wherein: the patient interface comprises any of the following:an unsealed nasal cannula, a sealed nasal cannula, a sealed nasal mask,or a full face mask; the gas source comprises any of the following: ablower, a flow meter, a flow mode from a ventilator, or a blender; orthe respiratory assistance system comprises a humidifier, the humidifierdisposed between the patient interface and the gas source, thehumidifier configured to humidify the gas from the gas source prior todelivery to the patient through the patient interface.
 14. The method ofclaim 1, wherein said plurality of test flow rate values are appliedresponsive to a user input.
 15. A method for delivering gas to a patientusing a respiratory assistance system comprising a patient interfaceconfigured to deliver a gas flow to the patient and a gas sourceconfigured to drive the gas flow to the patient interface at anoperating flow rate, said method comprising: using a hardware processor:applying a plurality of test flow rate values in a range as theoperating flow rate; measuring at least one patient parametercorresponding to each of the plurality of test flow rate values, whereinthe hardware processor waits a predetermined time period after a changein the operating flow rate before measuring the at least one patientparameter; determining a new flow rate value based at least in part onthe measured at least one patient parameter; and changing the operatingflow rate to the new flow rate value.
 16. The method of claim 15,further comprising determining a value in the range of the plurality oftest flow rate values at which the rate of change in the measured atleast one patient parameter approaches zero.
 17. The method of claim 15,wherein said determining the new flow rate value further comprisesdetermining a minimum value or a maximum value of the measured at leastone patient parameter for each of the plurality of test flow ratevalues.
 18. The method of claim 15, wherein the at least one patientparameter comprises a respiratory rate and wherein the method furthercomprises measuring the respiratory rate based at least in part on oneor more of the following measurements: pressure fluctuations, flow ratefluctuations, a blower fans speed, a blower motor power, a blower motortorque, expired CO2 fluctuations, transcutaneous CO2 fluctuations, anexpired patient temperature, EMG signals, Edi signals, impedancepneumography, respiratory inductance plethysmography, or acousticsensing.
 19. The method of claim 15, wherein the at least one patientparameter comprises one or more of a work of breathing indicator, anexpiratory CO2 concentration indicator, or a thoracoabdominal asynchronyindicator.
 20. The method of claim 19, wherein the work of breathingindicator comprises a respiration rate of the patient, wherein theoperating flow rate is selected such that the respiration rate of thepatient is minimized or reduced.
 21. The method of claim 15, wherein therange comprises one of the following: 0.5-1.5; 1-2; 1.5-2.5; 2-3; 0.5-4;1-3; 2-3; or 1-4 (in L/min/kg), or 5-120; 5-60; 20-120; 20-60; 60-120;or 50-70 (in L/min).
 22. The method of claim 15, wherein said changingthe operating flow rate comprises changing an amount of power deliveredto the gas source.
 23. The method of claim 15, wherein said determiningthe new flow rate value further comprises minimizing a work of breathingof the patient.
 24. The method of claim 15, further comprisinggenerating an alarm based on the measured at least one patientparameter.
 25. The method of claim 24, wherein said generating the alarmis in response to determining that the measured at least one patientparameter is insensitive to the change in the operating flow rate. 26.The method of claim 15, wherein: the patient interface comprises any ofthe following: an unsealed nasal cannula, a sealed nasal cannula, asealed nasal mask, or a full face mask; the gas source comprises any ofthe following: a blower, a flow meter, a flow mode from a ventilator, ora blender; or the respiratory assistance system comprises a humidifier,the humidifier disposed between the patient interface and the gassource, the humidifier configured to humidify the gas from the gassource prior to delivery to the patient through the patient interface.27. The method of claim 15, wherein said plurality of test flow ratevalues are applied responsive to a user input.
 28. A method fordelivering gas to a patient using a respiratory assistance systemcomprising a patient interface configured to deliver a gas flow to thepatient and a gas source configured to drive the gas flow to the patientinterface at an operating flow rate, said method comprising: applying aplurality of test flow rate values in a range as the operating flowrate; measuring at least one patient parameter corresponding to each ofthe plurality of test flow rate values; generating an alarm based on themeasured at least one patient parameter in response to determining thatthe measured at least one patient parameter is insensitive to a changein the operating flow rate; determining a new flow rate value based atleast in part on the measured at least one patient parameter; andchanging the operating flow rate to the new flow rate value.
 29. Themethod of claim 28, further comprising determining a value in the rangeof the plurality of test flow rate values at which the rate of change inthe measured at least one patient parameter approaches zero.
 30. Themethod of claim 28, wherein said determining the new flow rate valuefurther comprises determining a minimum value or a maximum value of themeasured at least one patient parameter for each of the plurality oftest flow rate values.
 31. The method of claim 28, wherein the at leastone patient parameter comprises a respiratory rate and wherein themethod further comprises measuring the respiratory rate based at leastin part on one or more of the following measurements: pressurefluctuations, flow rate fluctuations, a blower fans speed, a blowermotor power, a blower motor torque, expired CO2 fluctuations,transcutaneous CO2 fluctuations, an expired patient temperature, EMGsignals, Edi signals, impedance pneumography, respiratory inductanceplethysmography, or acoustic sensing.
 32. The method of claim 28,wherein the at least one patient parameter comprises one or more of awork of breathing indicator, an expiratory CO2 concentration indicator,or a thoracoabdominal asynchrony indicator.
 33. The method of claim 32,wherein the work of breathing indicator comprises a respiration rate ofthe patient, wherein the operating flow rate is selected such that therespiration rate of the patient is minimized or reduced.
 34. The methodof claim 28, wherein the range comprises one of the following: 0.5-1.5;1-2; 1.5-2.5; 2-3; 0.5-4; 1-3; 2-3; or 1-4 (in L/min/kg), or 5-120;5-60; 20-120; 20-60; 60-120; or 50-70 (in L/min).
 35. The method ofclaim 28, wherein said changing the operating flow rate compriseschanging an amount of power delivered to the gas source.
 36. The methodof claim 28, wherein said determining the new flow rate value furthercomprises minimizing a work of breathing of the patient.
 37. The methodof claim 28, wherein: the patient interface comprises any of thefollowing: an unsealed nasal cannula, a sealed nasal cannula, a sealednasal mask, or a full face mask; the gas source comprises any of thefollowing: a blower, a flow meter, a flow mode from a ventilator, or ablender; or the respiratory assistance system comprises a humidifier,the humidifier disposed between the patient interface and the gassource, the humidifier configured to humidify the gas from the gassource prior to delivery to the patient through the patient interface.38. The method of claim 28, wherein said plurality of test flow ratevalues are applied responsive to a user input.